In the related art, a position measurement can be made through various methods using GPS, RSSI (received signal strength indication), infrared rays, and ultrasonic waves.
Since the error of, for example, a position measuring technique using GPS is in the range of about 10 to 100 meters, it is difficult to apply the position measuring technique using GPS to application fields requiring precise position information.
In order to cope with such problems of the position measuring technique using GPS, GPS has been installed in a mobile phone for example, and various services using position information have been implemented. However, since GPS is usable only outdoors, it is difficult to accurately measure the ID and position of a person or object in a building.
Further, currently, it is possible to recognize a person or object indoors by techniques using an RFID tag, for example. However, it is possible to know that a tag exists in a RFID range, but it is not possible to know the precise position of the tag.
Similarly, a wireless local area network (WLAN) exhibits an error of several meters. A technique using ultrasonic waves has high accuracy and a narrow measurement range of several meters.
Recently, with the rapid development of sensor network techniques, the sensor network technique has been integrated with a technique for measuring the position of a moving object. Accordingly, objects of position measurement have been expanded from huge objects such as buildings and ships to small objects such as vehicles and persons. In other words, the number of objects being subjected to position measurement is increasing, and the sizes of the objects are decreasing. In keeping with this trend, research on techniques for recognizing the position of an object in real time and tracking the moving path of the object has been being carried out in many standardization organizations, and a representative example thereof is a WSN (wireless sensor network) in which a plurality of sensor nodes form a network in a wireless manner.
The WSN needs not only wireless communication and sensor network techniques but also various software techniques such as a routing protocol, and requires additional techniques to measure a position. For example, it is possible to measure the position of each moving node of the WSN by using RSSI (received signal strength indication). In this case, easy application to distance measurement is possible without additional hardware. An example of these application fields is a property position tracking system that finds the position of a sensor node attached to property, thereby finding the current position of the property.
Research on position tracking using a sensor network in various applications is being actively carried out. However, an attempt to seamlessly track the position of an object moving among many floors indoors has not yet been made.
FIG. 1 is a view schematically illustrating a general system for tracking a position in a building.
Referring to FIG. 1, a general system for tracking a position includes a moving-object tracking application 130, base stations 150, stationary nodes 173, and moving nodes 175.
The moving-object tracking application 130 provides a program for monitoring the positions of moving objects in real time, and tracks the moving nodes 175 by using position information on moving nodes 175 received through the base stations 150.
The base stations 150 are installed at every floor of the building. Each base station 150 receives position information and connection information from the moving nodes 175 distributed on a corresponding floor in real time, and stores the received information.
The stationary nodes 173 are installed on the walls or ceilings on individual floors, and serve as reference points required to calculate the positions of the moving nodes 175. In this case, the positions of the stationary nodes 173 are fixed at predetermined values.
Further, the stationary nodes 173 transmit or receive RSSI to or from the moving nodes 175, and provide the base stations 150 with basic position information that is used to calculate the positions of the moving nodes 175.
The moving nodes 175 are manufactured in a portable form and attached to property or persons that come in and out of the building. When the moving nodes enter WSNs managed by the base stations 150, the moving nodes transmit position information and connection information to the base stations 150.
Such a general system for tracking a position receives the position information and connection information, which are obtained by the stationary nodes 173 while the stationary nodes transmit and receive RSSI to/from the moving nodes 175, through the stationary nodes 173. Accordingly, the system can track the positions of the moving nodes 175.
Further, when a moving node 175 leaves a floor where the moving node is currently located (hereinafter referred to as a current floor) and arrives on another floor, a base station 150 of the current floor where the moving node 175 stayed receives connection release information on the moving node 175 from a WSN of the current floor, and thus recognizes that the moving node 175 no longer exists on the current floor.
Subsequently, a base station (not shown) of another floor where the moving node 175 has arrived receives connection set information on the moving node 175 from a WSN of another floor, and thus recognizes that the moving node 175 joins and is operating in the WSN of the other floor.
In a case where the connection release information on the current floor where the moving node 175 has stayed and the connection set information on the other floor where the moving node 175 arrives are sequentially transmitted, it is possible to track the precise position of the moving node 175. However, if a temporal difference where the connection set information is transmitted before the connection release information is transmitted due to variable conditions (for example, the density or number of a plurality of moving nodes, or interference) occurs, there is a problem in that the moving node 175 is temporarily recognized to be simultaneously located on a plurality of floors.
Therefore, in order to cope with the above-mentioned problem, a general WSN in the related art requires a method for preventing errors in position tracking caused by the temporal difference between a time point when the connection set information on the moving node 175 is transmitted and a time point when the connection release information on the moving node 175 is transmitted when the moving node 175 moves among a plurality of floors.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.